Define Observation or Inference for fehling's test?
An insoluble reddish brown precipitate of cuprous oxide will be obtained. The reddish brown precipitate indicates the presence of a reducing sugar. All monosaccharides are reducing sugars and give a positive Fehling's test.
Note- Sometimes you may see some yellow or greenish coloured precipitate.
Maltose and lactose have a free aldehydes group due to which they are capable of reducing Fehling's solution.
Sucrose does not possess any free sugar group since the aldehyde group of glucose molecule and the ketone group of fructose molecule are tied up in a glycosidic linkage. Hence, there is no free sugar group available for enediol formation and sucrose cannot reduce metallic ions.
Starch is made up of several glucose units linked by a-1,4 glycosidic linkages in straight chains and branched through -1,6 glycosidic linkages . Thus, it has very few free sugar groups and is not capable of reducing cupric ions in Fehling's reagent.
Dextrin is the intermediate product of starch hydrolysis and is a less complex molecule. Thus, it possesses more free sugar groups than starch and can partially reduce Fehling's reagent.